When using digital techniques in a phase-locked loop of a quadrature amplitude modulated (QAM) communication system, of the type shown and discussed in a U.S. Pat. No. 4,571,550 issued in the name of Manley J. Head and assigned to the same assignee as the present invention, it has been found that when the bit error rate exceeds a given value, some prior art out-of-lock detectors cannot distinguish the difference between in-lock with high bit error rate and out-of-lock conditions for the carrier phase-locked loop.
The present invention utilizes the most significant error bit and the next most significant error bit. Typically, this most significant error bit is called the error "sign" bit. These two bits also define the two most significant bits of lesser value than the data bits used to define the vector space for the data. By utilizing the information from these two bits in a logic array used to actuate an integrator type circuit, such as a counter, the circuit can tell with a high degree of accuracy whether or not the carrier phase-locked loop is out of lock, even in situations where there is a high bit error rate.
It is, therefore, an object of the present invention to provide an improved means for detecting out-of-lock conditions of a phase-locked loop in a QAM system.